"A commercial ZFS solution is (still) coming to Mac OS X, thanks to former Apple filesystem and OS engineer Don Brady (who previously worked on the abandoned internal Apple project to port ZFS). Brady and his company, Ten's Complement, just launched a limited private beta in hopes to have the software polished and ready for a summer launch this year. Ars spoke with Brady, who has a long history engineering filesystems for Mac OS and Mac OS X, to find out a little about his previous work with ZFS at Apple, and what Mac users can expect to gain from Ten's Complement's port of ZFS."

Another possibility is DragonFly BSD's HAMMER file system. It has some of the same features as ZFS but without the licensing issue. Apple has a lot of BSDL code in their tree.

BTRFS is feature-wise more-or-less on par with ZFS and thus could also make a suitable candidate. The current Linux version of it is however written in GPL and couldn't be used by Apple, and I doubt Apple will want to write a whole new implementation of it from scratch. :/